Copper foils for printed circuits commonly are laminated to a substrate of synthetic resin or the like at elevated temperature and pressure, printed with the pattern of a desired circuit, and unwanted portions are etched away. Finally, necessary elements are mounted or attached in place by soldering to form various printed circuit boards for electronic devices.
Qualitative requirements for the copper foil to be used in a printed wiring board vary with the sides of the resin substrate to which the foil is joined (so called "matte side" which has roughened surface) and not joined (so called "shiny side" which has glossy surface). It is important to satisfy the requirements for the both sides. Major requirements for the matte side are:
(1) No possibility of tarnishing with oxidation while in storage.
(2) Adequate resistance to peeling from the substrate even after high-temperature heating, wet treatment, soldering, chemical treatment, etc.
(3) Freedom from so-called stains or defect spots after lamination operation which may result from foil lamination to the substrate or from etching.
Conditions required of the shiny side include:
(1) Sound outward appearance and no tarnishing due to oxidation in storage.
(2) Good solder wettability.
(3) No oxidative discoloration on heating at elevated temperature.
(4) High adhesion to resist materials for producing printed circuits.
To meet these requirements, many different treatments have hitherto been introduced for the copper foils for printed wiring boards. The treatments differ with whether the foils are rolled or electrolytic. One process established as a useful approach consists of pretreating a degreased copper foil by plating, roughening, or the like as needed, alloy plating so as to produce a desired copper foil surface, treating the plated surface for corrosion prevention, and, where necessary, further treating the surface with silane coupling agent and further annealing.
The alloy plating is the key of the process which determines the surface properties of the resulting copper foil. The applicant has already proposed and gained some success in two treatments at typical alloy plating techniques. One was a Cu-Ni treatment (Japanese Patent Application Public Disclosure No. 145769/1987) and the other was a Cu-Co treatment (Japanese Patent Application Publication No. 2158/1988).
The Cu-Ni treatment imparts excellent peel strength (heat resistance and resistance to hydrochloric acid). On the other hand, the treated surface is difficult to etch, even with a copper chloride (CuCl.sub.2) etchant, and is not suitable for printed circuits of 150 .mu.m or finer pitches. Moreover, it cannot be etched with any alkali etching solution.
The Cu-Co treatment permits etching with a CuCl.sub.2 etchant to form 150 .mu.m or finer-pitch printed circuits and also allows alkali etching. The treated surface is inferior, however, in the point of peel strength (heat resistance and hydrochloric acid resistance) to the Cu-Ni-treated surface.
More recent tendencies toward finer patterning and diversification of printed circuitry have given rise to the requirements that
(i) peel strength (heat resistance and hydrochloric acid resistance) comparable to those of the Cu-Ni-treated be attained, and
(ii) etching with the CuCl.sub.2 etchant be possible to provide 150 .mu.m or finer-pitch printed circuits, and alkali etching too be feasible.
Specifically, the finer the circuit pattern the easier the circuit is to peel off by the action of the hydrochloric acid etchant, rendering it necessary to take some step against the peeling. The finer-patterned circuits also tend to be peeled off by the high temperature during soldering, again necessitating a preventive measure. With the tendency toward finer patterns, it is now imperative that etching with a CuCl.sub.2 etchant be possible to provide 150 .mu.m or finer-pitch printed circuits. The advent of new, diverse resists is also making alkali etching more and more essential.
Still another factor of importance (iii) is the magnetizability of the printed circuit. This is attracting increasing attention because the advent of higher-performance printed circuits and their expanding use, especially as FPC for magnetic heads, have more often designed the location of the circuits close to magnetic media than before. For such applications highly magnetizable alloys typified by conventional Cu-Co alloys cannot be used. Their saturation magnetization, remanent magnetization, and coercivity must be limited below predetermined levels.